That is not quite true. The advantages of LNG are much more important for high supersonic jets (Mach 2.5 and higher) than for subsonic jets. There are disadvantages too, and they are quite significant for all jets, but altogether the tradeoff is worth it at high speed long endurance supersonic jets.

Here's why. LNG offers 2 main benefits. The first is the higher energy density (53.6 MJ/kg vs 43 MJ/kg, so 25% more [1]). Airplanes are subject to the rocket equation, even if they are not rockets. The rocket equation says that the mass of the fueled vehicle is the mass of the vehicle at the end of the trip times the exponential of delta-v divided by the exhaust velocity. For airplanes, it is not exhaust velocity, but "effective exhaust velocity", because they borrow a lot of reaction mass from the atmosphere (the air used as oxidizer, and more importantly, the bypass air). The effective exhaust velocity is very high for subsonic airplanes, and much lower for high supersonic airplanes. The delta-v for subsonic airplanes is lower than the delta-v for supersonic airplanes because of the lower drag (although not as much lower as one would expect, because they need a higher attack angle). Overall, the benefit from the high energy density LNG is much more pronounced for high supersonic jets.

The second benefit is the use of the cryogenic LNG to cool off the engine. For very high speed engines, this is huge. So huge that the famous (but never materialized) SABRE engine was supposed to use liquid hydrogen, which is stored at much lower temperatures.

The disadvantage of LNG is, surprisingly, not the need for cryogenic storage. It is the lower volumetric energy density. It is 22% lower than that of jet fuel. The rocket equation does not care about volumes, only about mass, but larger volumes means bigger airplanes, so more drag.

So, for subsonic airplanes the advantages of LNG are not all that important, while the bulkier tanks are a pretty big downside. For high supersonic jets, the advantages of LNG are so high that they simply open up possibilities that are not there with jet fuel. The fact that the LNG is cheaper is a nice thing to have, but it's really not that important, since the economics of high supersonic jets are more impacted by the construction cost and very high maintenance cost than by the fuel cost.

[1] https://en.wikipedia.org/wiki/Energy_density#Chemical_reacti...

Surprisingly, at least in theory, and probably in practice with better technology, supersonic travel can be as efficient or even more efficient than subsonic flight. Supersonic travel opens up higher altitudes, higher altitudes means less air resistance.

The ultra high altitudes of LEO satellites showcase the steelman example, traveling effortlessly through the vanishingly thin atmosphere at hypersonic speeds with extreme efficiency even though the fuel expenditure to get them there was high.

For more reasonable hypersonic travel, at 100k feet, the “wind” force at 3375mph is only as much as you would feel at 400 mph at sea level… so you can exert the force needed to fly at 400mph, but for that same energy you are going 3375mph.

Of course there is a lot of tech needed to take advantage of these efficiencies, but it’s not a matter of faster = less efficient. As for economies, a jet that can fly LA to NY in 70 minutes, with an hour of turn at each end, could make 10 trips a day, potentially cutting the number of aircraft needed to cover a given route or route rotation by a factor of 4.

Obviously this is not currently practical on so many levels, but there is nothing fundamentally stopping us from achieving that level of service, given enough knowledge and technical capability.

If we ever want to achieve that level of understanding and competence, we will have to work on it when it seems impractical. Remember, it was in a single persons lifetime between flying precariously in glorified kites and supersonic flight.

> Airlines will always arbitrage toward the lowest energy-per-seat-km for most routes

That's a second order effect from fuel being the primary cost, and thus the primary lever to either make more profit or improve competitivity.

If airlines could triple their profits by doubling their fuel burn they'd happily do that.

I agree with your market analysis. Private jets are often referred to as "time machines" given how much time HNW / exec travelers can save. There's a market segment that's willing to pay a high premium for reduced travel time.

Hm... I don't know that I buy your argument, since just as you point out, traditional jets are already very optimized. One would assume there's less slack to pick up.

Aeronautical engineering isn't that linear. A technology suited to one application may not be helpful in another. It's one of those "hardware is hard" fields.

> Historically, faster transport doesn’t replace slower transport wholesale; it creates a premium tier while pushing the mass market down to a lower cost/energy equilibrium.

If that were true, we’d all be taking trains and boats everywhere. We aren’t.

You are missing an important factor in the baseline here, the cost of time.

Right now, a cheap 7 hour each way round trip between NYC and London is ~500$.

Halve it to 3.5 hours each way with a supersonic plane, saving a total of 7 hours.

Now, the real question is then, what's one hour of your time worth to you or whoever is paying for your flight?

If improvements to subsonic aircrafts bring down the price to 200$ instead of $500, people would still be willing to pay 200$ + 7 * $HOURLY for a faster flight.

Even with a low-ish estimate of $HOURLY = 50, it would make sense to take the supersonic fight if the price was $500, which it could conceivably be brought down to, and the market has already validated to be willing to pay.

And even then, it is only so premium. As you could have a speedy economy seat on the Concorde or a lie flat bed on a widebody by the time Concorde left service. The speed benefit largely goes away if I can travel while sleeping.

Want to reduce the time it takes to get somewhere? Reduce the security circus at airports. This will cut off way more of the travel time for the majority of flights, wothout the downsides of supersonic planes.